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Fix/merge advice on low percentage progress bars

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- actually add the ``min-width`` extra style (and use ``em`` rather than
``px``) to the 0% and 2% example
- remove the unnecessarily redundant sentence in the "With labels"
section
- remove the unnecessary heading/section specifically for low percentage
case, make it flow naturally as part of the "With labels" section

Tie help block to input with aria-describedby

Add grunt-contrib-compress.

Now, the zip file is created when running `grunt prep-release`.

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More docs accessibility tweaks

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unless there's a reason why ``tabindex="-1"`` was dropped in there...

Cleaner alternative to using ``<span class="sr-only">``

plus a few suggested approaches (in some cases, elided, as the actual
content of the label - being a generic example - is not important)

to clarify/hint that a ``<label class="sr-only">`` is not the only way
to do it

Re-add missing "to" in docs

Docs accessibility tweaks: use ``aria-label`` for close ``<button>``, expand advice for ``<abbr>``, form control labels, make sidebar navigation a ``<nav>``

...and remove the redundant paragraph for basic abbreviation, which
simply repeats what was just said above.

Harmonizing this with same advice given for CSS inline forms

shorter, neater, well supported

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Catch and properly report compile errors that Less.js's toCSS() can throw

Currently these errors are only reported in the JavaScript console, not in the Customizer UI.
Refs #15296

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Refs #15238
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Update source maps info in README.md.

Remove execute permission on fonts.

Give all embedded live docs examples data-example-id attrs
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This reverts commit 2c1c885e.